The study of helicity upon light-matter interactions took a clear boost in 2013 when it was theoretically
unveiled that helicity can be conserved upon scattering for dual materials, regardless of their geometry [1]}. A dual material is such that the ratio between its relative magnetic permeability and its electric permittivity happens to be constant (\(\mu /\varepsilon = \text{constant}\) ). These materials restore duality symmetry in the macroscopic approximation of Maxwell equations. Unfortunately, there are no magnetic materials (\(\mu \ne 1\) ) at optical frequencies. The implication is that it should be impossible to experimentally observe helicity conservation at optical frequencies. However, shortly after, it was demonstrated [2]} that high refractive index particles could approximately conserve helicity due to their electric and magnetic resonances [3]}, [4]}. Since then, a lot of work has been done to characterize the role of helicity in light-matter interactions [5]}, [6]}, [7]}, [8]}, as well as to study its relation with chirality [9]}, [10]}, [11]}, [12]}.
